Multimode optical fibers consist of glass fibers that are typically in the range from about 100-300 microns in diameter. The length of such fibers can be from several meters to several kilometers. Light can be guided through the interior of such fibers by manufacturing them with a core of glass having a high index of refraction, and an exterior cladding of glass having a low index of refraction. Where two discrete types of glass are used, the fiber is said to be of a step index type. Where there is a gradual decrease in the refractive index from the inside to the outside of the fiber, the fiber is of the graded index type. In both instances, however, the outer most layer of glass, i.e., the cladding, is typically tens of microns thick.
To shield the light from being absorbed by dirt or other objects on the fiber surface, especially over long lengths, the cladding is kept fairly thick. For instance, in a graded index fiber of the Corning Company, the core diameter is 62.5 microns with a cladding thickness of 31.25 microns. Thus, the total fiber diameter is 125 microns (core plus two thicknesses of cladding).
A good review of optical fibers can be found in the textbook "Fibre Optics, Theory and Practice"; by W. B. Allan, 1973 Plenum Press, London.
An optical fiber coupler can be considered an optical device which shares the light travelling down one fiber with at least one other fiber. Such a device is essential for communications by optical fibers since there are many instances when one wishes to access the data carried by a main fiber, or to send data from a secondary fiber to a main or trunk fiber. For example, in cable television applications the main trunk fiber, which carries the television signals, would be connected by subsidiary fibers to each household. Each junction of a subsidiary fiber with the main fiber would require an optical fiber coupler.
It will clearly be recognized that the less light that is wasted by an optical fiber coupler, the better will be the communications system using that coupler.
One prior art optical fiber coupler of which we are aware comprises two fused fibers which are also biconically tapered in the fusion region. That design was concerned primarily with the biconical tapering and twisting of the two fibers during fusing. Any modifications or analysis with respect to the thickness of the cladding were not considered and are not encompassed in that earlier design. This earlier design is described in the following:
1. Paper by T. Ozeki and B. Kawasaki in Applied Physics Letters, Vol. 28, P. 528, 1976, entitled "Optical Directional Coupler Using Tapering Section in Multimode Fibers".
2. Paper by B. Kawasaki and K. Hill in Applied Optics, Vol. 16, P. 1794, 1977, entitled "Low Loss Access Coupler for Multimode Fiber Distribution Networks".